I would go with C2D. Quad Core CPU's, while sexy, won't be cost effective for at least 1-2 years IMO. Similar to 64-bit processors when they first came out, there just aren't any applications/programs that will be able to take advantage of all 4 cores.
i wouldn't be utilizing 4 cores on anything but high definition video, however. most encoders use 4 threads via slices, output will take a compressibility hit with every core. imo no more than two at dvd resolutions
Don't get me wrong, I agree that Quad Core is on the higher end (obviously).
My only concern is cost difference (not sure off top of my head) versus performance gain. I don't do a whole lot of encoding, hardly any at all, but I would think the difference in cost versus what is gained just isn't worth it until the actual software can take full advantage of the quad cores.
I'd like to see some real world benchmarks though comparing the two. Seem to remember reading an article saying that the C2D EE QX6800 is better than some of the Quad Cores, will try to find that article if I remember where I saw it...
they also have a touch more brute force than the duo's. They are based on the same core (if you get xeon 5000 series) but with the increased cache and FSB clocks and the ability to put more than one physical chip on board your only limit on processing muscle is your budget.
IF your budget doesn't reach that far then a core 2 duo or quadro on a 64bit os with 64bit encoders, should let you steam through most encoding without breaking a sweat
Heck a core 2 duo can encode a dvd, decode a dvd and burn a dvd and only hit 69% total cpu usage according to task manager
odd, since dvd shrink is multi-threaded, it should use at least 90%+ of your cpu, and close to 100% during deep analysis. but it would depend on source & destination drives or hard disk load, and amount of compression applied.
the new Quad coming out next year - if you can afford it - should achieve at least 30% increases on the fastest duos you can get now.... So I think I would wait for that (there's something quite exciting about quad to me... not sure why....)